Fiber coating compounds for reinforced ceramic matrix composites

ABSTRACT

A composite ( 20 ) has a matrix ( 10 ), preferably of ceramic, interspersed with reinforcement structures ( 12 ), preferably ceramic fibers, coated with a material ( 14 ) selected from ZrGeO 4 , HfGeO 4  and CeGeO 4 , where the composite can be used as a component in high temperature turbines.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to high temperature capableceramic matrix composites (CMCs), and more particularly to theirapplication in gas turbines.

2. Background Information

Combustion turbines comprise a casing or cylinder for housing acompressor section, combustion section and turbine section. Thecompressor section comprises an inlet end and a discharge end. Thecombustion section or combustor comprises an inlet end and a combustortransition. The combustor transition is proximate the discharge end ofthe combustion section and comprises a wall which defines a flow channelwhich directs the working fluid into the turbine section's inlet end.

A supply of air is compressed in the compressor section and directedinto the combustion section. Fuel enters the combustion section by meansof a nozzle. The compressed air enters the combustion inlet and is mixedwith the fuel. The air/fuel mixture is then combusted to produce hightemperature and high pressure gas. This working gas is then ejected pastthe combustor transition and injected into the turbine section to runthe turbine.

The turbine section comprises rows of vanes which direct the working gasto the airfoil portions of the turbine blades. The working gas flowsthrough the turbine section causing the turbine blades to rotate,thereby turning the rotor, which is connected to a generator forproducing electricity.

As those skilled in the art are aware, the maximum power output of acombustion turbine is achieved by heating the gas flowing through thecombustion section to as high a temperature as is feasible. The hot gas,however, heats the various turbine components, such as the combustor,transition ducts, vanes and ring segments, that it passes when flowingthrough the turbine.

Accordingly, the ability to increase the combustion firing temperatureis limited by the ability of the turbine components to withstandincreased temperatures. Consequently, various cooling methods have beendeveloped to cool turbine hot parts. These methods include open-loop aircooling techniques and closed-loop steam cooling systems. Bothtechniques, however, require significant design complexity, haveconsiderable installation and operating costs and often carry attendantlosses in turbine efficiency.

In addition, various insulation materials have been developed tostrengthen the resistance of turbine critical components to increasedtemperature. Thermal Barrier Coatings (TBCs) are commonly used toprotect critical components from premature breakdown due to increasedtemperatures to which the components are exposed. Generally, TBCs extendthe life of critical components by reducing the rate of metal waste(through spalling) by oxidation.

In Advanced Turbine Systems (ATSs), however, the temperature demands ofoperation and the limits of ATS state-of-the-art materials, may lead tofailure of the TBCs. This, in turn, can result in premature failure ofthe critical components and therefore, possible failure of the turbine,interruption in the power supply and expensive repair costs. It is,therefore, desirable to provide turbine components that can withstandhigh temperatures without the use of thermal barrier coatings and reducethe need for cooling.

Commercially available oxide fiber/oxide matrix ceramic matrixcomposites (CMCS) have many potential applications in gas turbines, butare limited in their exposure to temperatures near 1000-1100° C. forlong periods of time, that is, greater than 10,000 hours for gasturbines used in power generation. In addition, CMCs cannot beeffectively cooled under high temperature conditions (greater than 1400°C.) or high heat flux conditions due to their relatively low thermalconductivity and inability to fabricate intricate cooling passages.Furthermore, while commercially available CMCs already have goodtoughness, strain and damage tolerance, improvements in these areas area continual goal for the development of CMCs.

Combustion of the fuel/air mixture occurs at temperatures much higherthan the melting point of the metallic combustor liner. For this reason,the liners must be cooled by non-combusted, cooler air and are usuallycoated with thermal barrier coatings. The most common way of coolingmetallic liners is by way of film cooling, which introduces cool airthrough the wall of the liner by way of small holes drilled at an acuteangle to the surface. This air, in turn, forms a cooler boundary layeron the inside surface of the combustor liner, protecting it from the hotcombustion gases. One of the problems with film cooling is thatundesirable combustion byproducts (carbon monoxide and unburnedhydrocarbons) occur when the cooler air mixes with the hot gases. Inanticipation of dilution due to film cooling, the fuel/air mixture isconsequently richer than desirable, resulting in excessive NO_(x)emissions. A true hot wall combustor requires no film cooling (resultingin lower carbon monoxide and unburned hydrocarbons emissions), allowsleaner combustion (resulting in lower NO_(x) emissions), and providesincreased flame stability (resulting in greater durability andreliability).

The transition duct is a large, complex structure which contains the hotcombustion gases and directs them into the turbine inlet. The largesurface area and the high internal temperature make these partsextremely difficult to cool effectively. Conventional transitions aremade from nickel-based superalloys coated internally with thermalbarrier coatings. The latest high efficiency utility engines necessitatethat these parts be actively cooled, requiring internal wall coolingpassages, and complex and costly construction. With much simplerconstruction, lower cost components would be possible using an insulatedCMC concept. Passive cooling methods could be employed using redirectedcombustor inlet gases, resulting in net efficiency gains.

The first stage of turbine vanes direct the combustion exhaust gases tothe airfoil portions of the first row of rotating turbine blades. Thesevanes are subjected to high velocity, high temperature gases under highpressure conditions. In addition, these are complex parts with highsurface areas and, therefore, are difficult to cool to acceptabletemperatures. Conventional state-of-the-art first row turbine vanes arefabricated from single-crystal superalloy castings with intricatecooling passages and with external thermal barrier coatings applied. Notonly are these components expensive to manufacture, but withever-increasing gas path temperatures, their ability to be effectivelycooled is limited. Higher temperature materials would obviate the needfor such complexity, thus minimizing cost, and also minimizing the needfor cooling air, thereby improving engine efficiency and reducingoperating costs.

The rotating turbine or rotor of an axial flow gas turbine consists of aplurality of blades attached to a rotor disk. In operation, the shaftand blades rotate inside a shroud. Preferably, the inner surface of theinner wall of the shroud is coated with an abradable material. Theinitial placement of the rotor blades are such that the blade tips areas close as possible to the coating. Improvements in this essentialcomponent are also desirable.

U.S. patent application Ser. No. 09/049,369, filed on Mar. 27, 1998(Attorney Docket No. T2 97-26; Morrison et al.) attempted to solve manyof the above problems by providing a new and improved ceramiccomposition that could be bonded to standard ceramic matrix composites,to provide a hybrid-CMC. The ceramic composition used there contained aplurality of hollow, oxide-based spheres of various dimensions, aphosphate binder, and a solid oxide filler, where the binder filled gapsbetween the hollow spheres and the filler. Each sphere was situated suchthat it was in contact with at least one other sphere. The ceramiccomposition acted as a high temperature insulator, which was alsoabradable, to be applied over the higher strength, lower temperatureresistant CMC. The two could be mated with a thin layer of aluminumphosphate-based or alternative high temperature adhesive. The CMC usedcould be made of oxide matrix composites (such as mullite,aluminosilicate or alumina), silicon carbide matrix composites, orsilicon nitride matrix composites. The CMC used could be in the form ofturbine blades, combustors, transition ducts, vanes, ring segments, andthe like, with the ceramic composition bonded to their surfaces. U.S.Pat. No. 6,013,592 (Merrill et al.) is a similar patent in this area.

Fiber, whisker and particulate reinforced thermodynamically-stable hightemperature ceramic matrix composites are also known in the art, such asthose taught in U.S. Pat. No. 5,514,474 (Morgan et al.). There, a weakinterface coating between the reinforcement fibers and the ceramicmatrix was thought desirable, to allow debonding and sliding of thereinforcement if a crack were to impinge upon it from the matrix. Theinterface coating in the fibers was a monazite, MPO₄, where M was alarger trivalent rare earth of the lanthanide series, such as La, Ce,Pr, etc.); alternately, the interface coating was a xenotime, MPO₄,where M was a smaller trivalent rare earth of the lanthanide series,such as Sc, Y, Dy, Ho, etc. In the examples, the reinforcing fibers,such as thin sapphire fibers, were coated with a LaPO₄ coating, using aslurry dipping process. Al₂O₃, MgAl₂O₄, ZrO₂, SiC and mullite fiberswere also disclosed.

A similar fiber reinforced composite was described in U.S. Pat. No.5,759,632 (Boakye et al.). There, however, the interface coating—amonazite—was applied by a sol-gel dipping technique. Another usefulfiber coating interlayer for CMCs is CaWO₄, as demonstrated in“Isotropic Complex Oxides as Fiber Coatings for Oxide/Oxide CFCCS” (R.W. Goettler et al., Ceramic Engineering and Science Proceedings, vol.18, no. 3, 1997, pp. 279-286.)

Karlin et al., in “Phase Diagram, Short-Range Structure, and AmorphousPhases in the ZrO₂−GeO₂(—H₂O) System” (Journal of the American CeramicsSociety, vol. 82, no. 3, 1999, pp. 735-741), discussed the processing of(1−x)ZrO_(2−x)GeO₂. According to the article, preparation of glasses andceramics via the sol-gel route allowed preparation of: high-puritymaterials, glasses from reluctant glass-forming materials, homogeneouslydispersed nanocomposites, and thin coatings, particularly for fiberreinforced composites. There, the use of long ceramic fibers embedded ina refractory matrix, resulting in a tough pseudo-plastic material havingimportant uses as advanced engine composites, was discussed. ZrO₂ wasidentified as an appropriate interface coating for thermally stableCMCs. Phase relationships were studied, and thermogravimetric andshrinkage analyses were performed.

New interface coating systems are needed at the fiber/matrix interfacein CMCs, in order to allow fabrication of increasingly damage tolerantand more thermally stable CMCs for components used in gas turbineoperations. CMCs are needed which require reduced cooling air flow andwhich are made with relatively inexpensive and commercially availablematerials. Such CMCs should also be useful in combination with a ceramicinsulating layer, to provide longer-life hybrid-CMC components.

SUMMARY OF THE INVENTION

Thus, it is a main object of the invention to provide a ceramic matrixcomposite (CMC) having high temperature and damage tolerance capability,for use in a gas turbine environment, comprising a suitably coatedparticulate or fibrous material in a ceramic matrix.

It is another main object of this invention to provide a ceramic matrixcomposite and a hybrid ceramic matrix composite which require reducedcooling air flows and which are made with relatively inexpensive andcommercially available materials.

These and other objects of this invention are accomplished by providinga composite having high temperature and damage tolerance capability,comprising a matrix material having interspersed therein solidreinforcement structures coated with an interface coating materialconsisting essentially of scheelite germanates of the formula MGeO₄,where M consists of elements selected from the group consisting of Zr,Hf, Ce and mixtures thereof, where the coating provides a chemicalreaction barrier and a weakly-bonded interface surface between thereinforcement structures and the matrix.

Preferably, the composite is a ceramic matrix composite (CMC), where thereinforcement structures within the matrix material are fibers. Thesecomposites have a “high temperature capability” of up to at least 10,000hours at 1100° C. They have “damage tolerance” in that the interfacecoating material can fracture or slip within the matrix in response tostress and can deter the propagation of matrix cracks. The interfacecoating is a “chemical barrier” which will prevent chemical reactionsbetween the matrix and the fibers. The composite can also have anattached additional abradable insulating structure containing hollow,oxide-based spheres in a suitable heat resistant (up to 1300° C. orgreater) binder, to form a hybrid-CMC. Hybrid-CMCs are especially usefulas hot gas path turbine components (such as combustors, transitions,vanes, ring segments and turbine blades) for advanced gas turbinesoperating in a hot gas environment up to 1600° C. Preferably, both thereinforcement structures and the matrix are refractory oxide ceramics,such as alumina, mullite and yttria-alumina-garnet (YAG).

The CMCs of this invention, with the described scheelite germanateinterface coatings, have the potential to satisfy the requirements oftoughness, damage tolerance, thermal stability and long life (at least10,000 hours), for use as hybrid-CMC hot gas path components in advancedgas turbine engines. Hot gas path components based on the hybrid-CMCconcept and using the specialized CMCs of this invention willdemonstrate a significant decrease (about 90%) in cooling airrequirements, as compared to the current state-of-the-art metalcomponents, resulting in an overall increase in efficiency of 1.5 to 2.0percentage points. Scheelite germanate fiber coatings provide thefollowing advantages over current oxide/oxide CMC technology: a reactionbarrier between the fiber and the matrix; fiber/matrix sliding straintolerance mechanisms to increase damage tolerance; less environmentalexposure for the fibers to increase CMC life; and improved matrixdominated properties, for example, greater thermal conductivity andgreater off-axis, compressive and interlaminar strength.

BRIEF DESCRIPTION OF THE DRAWING

The above and other advantages of the invention will be more apparentfrom the following description, in view of the drawing, in which:

FIG. 1 is a simplified cross-section showing the reinforced ceramicmatrix composite of this invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the Figure, a ceramic matrix composite 20 is shown,having a matrix 10 (preferably of ceramic) with interspersed solidreinforcement structures 12, having approximate diameters of from 2.5micrometers to about 100.0 micrometers or greater. For the sake ofclarity, only a small number of reinforcement structures are shown inthe given matrix area in the drawing. These reinforcement structures canbe solid spheres, but most preferably are long whiskers or fibers (here,all shown in cross-section). Preferably, the matrix 10 and reinforcementstructures 12 are over 70% of theoretical density, most preferably from90% to 100% of theoretical density. At composite densities over 80%, itis more difficult to volatilize the coating 14 (described below). Thereinforcement structures 12 (hereinafter referred to as fibers, for thesake of convenience), are coated with a material 14 consistingessentially of ZrGeO₄, HfGeO₄, CeGeO₄, and mixtures thereof. Thiscoating must have a thickness Y ranging from 0.02 micrometers to 2.0micrometers, preferably from about 0.04 micrometers to 1.0 micrometers.The coating is preferably 80% to 100% of theoretical density, to improvethe durability of the coating during handling and during CMC processing.In some instances, as shown at 22, the coating may be discontinuous; andin some instances, as shown at 24, the reinforcement structures maycontact one another. The combined amount of reinforcement structure andcoating is from about 30 vol. % to about 50 vol. %. A crack 16 is shownpropagating through the matrix 10, where it is deflected by the coating14, which creates a weak bond between the fiber 12 and the matrix 10 attheir interface 18, so that the fiber can slip within the matrix toaccommodate the strain.

Many prior art structures have relied upon porous (less than 70% oftheoretical density) matrices to supply damage tolerance. Such prior artstructures compromised the mechanical (off-axis, compressive andinterlaminar strengths) and thermal (conductive) properties of the CMC.Many CMCs relied on non-oxide fiber/matrix interface materials (such aspyrolitic carbon or hexagonal boron nitride), which possess goodmechanical properties, but are subject to rapid oxidation in hightemperature oxidizing environments.

The interface coating material used in sophisticated CMCs should protectthe ceramic fiber during matrix processing and prevent reactions betweenthe fiber and the matrix during long-term high temperature exposure, andshould provide a week fiber/matrix interface, allowing processing of amatrix with a higher relative density, resulting in improved mechanicaland thermal properties of the CMC. Furthermore, the deposition of thefiber coating should not degrade the properties of the ceramic fiber.Therefore, deposition should be performed at sufficiently lowtemperatures using precursors that are not corrosive to the fibers.

In this invention, a single layer scheelite germanate fiber coating isused in CMCs for advanced gas turbine applications. Scheelite germanatecompounds include ZrGeO₄, HfGeO₄, and CeGeO₄. This invention encompassesscheelite slurry infiltration of the ceramic fiber tows and/or performsor chemical vapor deposition on the ceramic fibers, among othertechniques. These fiber coating compounds are intended for use onalumina, mullite, and YAG fibers; however, these compounds could be usedas fiber coatings for fibers of any composition. These scheelitegermanate oxide ceramic coatings have the following characteristics:they are oxidation resistant; they provide a layered crystal structureand anisotropic thermal expansion; and they are relatively easily formedfrom simple oxides due to their low melting temperature. They are alsominimally reactive with alumina, mullite and YAG fibers. It is believedthat a hybrid-CMC formed from the CMC of this invention, in combinationwith an additional abradable insulating structure (such as previouslydescribed), would result in over 90% cooling air savings and about 2%improvement in heat rate, as well as reduced emissions, compared to acorresponding metal component.

The scheelite germanates used in this invention have a scheelite crystalstructure and are relatively weak compounds. The scheelite crystalstructure is layered. In general, the greater the degree of layering inthe crystal structure, the weaker the interface material. The degree ofthermal expansion anisotropy gives an indication of the degree oflayering in the crystal structure. Table 1 shows the coefficient ofthermal expansion (CTE) anisotropy of CaWO₄ and the scheelitegermanates. The greater the percent (CTE) anisotropy, the weaker theinterface material which provides a more damage tolerant composite.

TABLE 1 Thermal Expansion Anisotropy of Scheelite Compounds¹ CompoundCTE: c-axis CTE: a-axis Percent Anisotropy CaWO₄ 11.5 19.2 57 ZrGeO₄ 4.98.6 73 HfGeO₄ 4.2 7.1 63 CeGeO₄ 4.6 9.6 117  ¹From G. Bayer et al.,“Thermal Expansion of ABO₄ Compounds with Zircon and ScheeliteStructures”, Journal of Less Common Metals, vol. 26, 1972, pp. 255-262.

Scheelite germanates have CTEC ranging from about 3.5 to about 10. Thepercent anisotropy was calculated as follows: percentanisotropy=([c:axis-a:axis]/a:axis)(100). CaWO₄ has been demonstrated asan effective fiber/matrix interface material in oxide/oxide CMCs. (See,for example, the article by R. W. Goettler et al., describedpreviously.) In terms of crystal structure, the scheelite germanates arepreferred over CaWO₄, because of their greater CTE anisotropy and theirlower overall CTE, as noted in Table 1 (above).

Another characteristic, important to the coatings of this invention, isthe ease of forming the scheelite gernanate compounds. Based on hightemperature phase stability with alumina, a ZrO₂ fiber coating is anexcellent choice for alumina-based CMCs. When ZrO₂ fiber coatings areproduced from sols, the crystallization temperature is low (about 450°C.) and the sintering temperature of the crystallized phase is greaterthan the temperature capability of currently available polycrystallineoxide fibers. Thus, it is difficult to form dense, adherent ZrO₂ fibercoatings from a zirconia sol. The co-deposition of zirconia and germaniasols significantly increases the crystallization temperature (to greaterthan 800° C. with a 50:50 mole ratio of ZrO₂ to GeO₂) and decreases thesintering temperature. Since the melting temperature of GeO₂ isrelatively low (only 1086° C.), a viscous phase can be reached toenhance greatly the densification of the fiber coating. Related to thelow melting temperature, the evaporation temperature of GeO₂ is also low(1550° C.), and the vapor pressure of GeO₂ is relatively high attemperatures above 1000° C. Consequently, heating could result in theevaporation of GeO₂ from a GeO₂-rich phase, thus causing the coating totransform to a refractory ZrO₂-rich phase.

The low melting temperature and high vapor pressure at temperatures ofgreater than 1000° C. can be a potential problem of using germanatecompounds as fiber/matrix interface coatings for high temperatureoxide/oxide CMCs. However, we have found that there are a number ofmitigating factors which reduce the apparent disadvantages of germanatecompounds. One mitigating factor is that the effectiveness of germanatecompounds is not strongly dependent on stoichiometry. Zirconiumgermanate forms two compounds with a scheelite crystal structure, ZrGeO₄and Zr₃GeO₈. Loss of GeO₂ from ZrGeO₄ (such as described above) causesthe transformation to the more refractory Zr₃GeO₈ phase. Further loss ofGeO₂ from Zr₃GeO₈ results in a highly refractory, porous ZrO₂ phase,which still provides an advantageous coating. Thus, the potential lossof GeO₂ does not seriously challenge the effectiveness of the coatings.

Another mitigating factor of using germanate compounds is that thetemperature gradient in a minimally cooled, thermally insulatedstructural CMC (that is, a hybrid-CMC) reduces the potential loss ofGeO₂. Only in a top portion 26 near the hot surface 28 will the CMC behot enough to volatilize GeO₂ and ultimately form a refractory ZrO₂interface. In a second portion 30 near the cooler surface 32 of the CMC,the ZrGeO₄ phase should remain at the interface. A third mitigatingfactor which may reduce the effect of GeO₂ volatility is the density ofthe matrix. If the ZrGeO₄ interface is imbedded in a matrix which ishighly dense (greater than 80% of theoretical density), volatilizationof GeO₂ should be slow.

A further advantage of the scheelite germanates is that ZrGeO₄, HfGeO₄,and CeGeO₄ compounds are minimally reactive with alumina, mullite andYAG fibers. This is important because strength retention of the fibersduring long-term thermal exposure is critical. The scheelite germanatesare more benign to those fibers than many of the coating compoundsdescribed in the prior art. For example, the cations of the scheelitegermanates (ZrO₂, HfO₂ and CeO₂) are more phase compatible with aluminathan the cation of CaWO₄ (CaO). Also, it is believed that the anion,GeO₂, does not react with mullite at or below 1100° C., which is nearthe maximum temperature capability of current fibers (which isapproximately 1200° C.). Conversely, excess phosphate (P₂O₅) in monazite(LaPO₄) or xenotime (YPO₄) would likely react with alumina or mullite.

The present invention may be embodied in other forms without departingfrom the spirit or essential attributes thereof, and accordinglyreference should be made to both the appended claims and the foregoingspecification as indicating the scope of the invention.

What is claimed is:
 1. A composite having high temperature and damagetolerance capability, comprising a matrix material having interspersedtherein solid reinforcement structures coated with an interface coatingmaterial consisting essentially of scheelite germanates of the formulaMGeO₄, where M consists of elements selected from the group consistingof Hf, Ce and mixtures thereof, where the coating provides a chemicalreaction barrier and a weakly-bonded interface surface between thereinforcement structures and the matrix.
 2. The composite of claim 1,where the matrix is ceramic.
 3. The composite of claim 1, where thereinforcement structures are ceramic fibers.
 4. The composite of claim1, used as a turbine component.
 5. The composite of claim 1, used as aturbine component selected from the group consisting of combustors,transitions, vanes, ring segments, turbine blades, and the like.
 6. Thecomposite of claim 1, having the ability to withstand temperatures of atleast about 1100° C. for at least 10,000 hours.
 7. The composite ofclaim 1, where the interface coating material can slip within the matrixin response to stress and can deter the propagation of matrix cracks. 8.The composite of claim 1, where the reinforcement structures are fibersmade of a material selected from the group consisting of alumina,mullite and yttria-alumina-garnet.
 9. The composite of claim 1, wherethe matrix is made of a material selected from the group consisting ofalumina, mullite and yttria-alumina-garnet.
 10. The composite of claim1, having an additional attached abradable insulating structurecontaining hollow, oxide-based spheres within a heat resistant binder.11. The composite of claim 1, where the matrix has a density of over 70%of theoretical density.
 12. The composite of claim 1, where the combinedamount of reinforcement structure and coating is from about 30 vol. % toabout 50 vol. %.
 13. The composite of claim 1, where the coatingmaterial has a coefficient of thermal expansion from about 3.5 to about10.
 14. A composite material having high temperature and damagetolerance capability, the composite material comprising: a matrixmaterial comprising a first portion proximate a surface and a secondportion below the first portion opposed the surface; a reinforcementstructure disposed within the matrix material and extending into boththe first portion and the second portion; a first interface coatingmaterial comprising one of the group of ZrO₂ and a first phase ofscheelite germanate disposed between the reinforcement structure and thefirst portion of the matrix material; and a second interface coatingmaterial comprising a second phase of scheelite germanate different thanthe first phase disposed between the reinforcement structure and thesecond portion of the matrix material.
 15. The composite material ofclaim 14, wherein the second interface coating comprises a GeO₂-richphase of a scheelite germanate and the first interface coating comprisesa refractory phase of the scheelite germanate formed by evaporating GeO₂from the scheelite germanate.
 16. The composite material of claim 14,wherein the first interface coating comprises a refractory materialphase.
 17. The composite material of claim 14, wherein the firstinterface coating comprises ZrO₂ and the second interface coatingcomprises ZrGeO₄.
 18. A composite material having high temperature anddamage tolerance capability, the composite material comprising: a matrixmaterial; a reinforcement structure disposed within the matrix material;an interface coating material disposed between the reinforcementstructure and the matrix material, the interface coating comprising alayered crystal structure having a thermal expansion anisotropy ofgreater than 73%, wherein the interface coating material comprisesCeGeO₄.
 19. A ceramic matrix composite material comprising: a ceramicmatrix material; an oxide ceramic reinforcement structure dispersedwithin the matrix material; and an interface coating between the ceramicmatrix material and the oxide ceramic reinforcement structure, theinterface coating comprising a refractory phase of zirconium germanatecomprising one of the group of Zr₃GeO₈ and a porous ZrO₂-rich phase. 20.The material of claim 19, wherein the interface coating comprisesZr₃GeO₈.
 21. The material of claim 19, wherein the interface coatingcomprises a porous ZrO₂-rich phase of zirconium germanate.